1. Field of the Invention
The present invention relates to new polypeptides that have the capability of absorbing large amounts of water and aqueous solutions, in particular physiological saline solutions, and methods for preparing such polypeptides. These polymers are useful in a variety of applications including but not limited to sanitary goods, hygienic goods, water retaining agents, dehydrating agents, and control release agents for various chemicals.
2. Discussion of the Background
In general, superabsorbent polymers possess a structure in which the water-soluble polymer has been made insoluble by some process, typically by means of a cross-linking agent, resulting in polymers that have the power to absorb at least 20 times their weight in pure water. Water absorbing resins currently in use include hydrolysis products of starch-acrylonitrile graft polymers, carboxymethylcellulose, polycarboxylic acids, acrylamides, non-cross-linked polymer blends, cross-linked polyacrylate products and other resins such as polyvinyl alcohols (Mikita et al, U.S. Pat. No. 4,703,067; Ofstead, R. F., U.S. Pat. No. 4,771,089; and Siddall, J. H. et al, U.S. Pat. No. 4,833,224). Brandt et al, U.S. Pat. No. Re. 32,649 also disclose hydrogel-forming polymer compositions based on polymerized unsaturated polymerizable acid group-containing monomers and a cross-linking agent. Superabsorbent polymers are polyanionic in nature and it is the hydration of these charged groups that leads to the absorbent characteristics (Masuda, F., "Super absorbent polymers--characteristics and trends in development of applications," Chem. Econ. Enginer. Rev., vol. 15, pp. 19-23 (1983)).
One problem with such resins is that the absorptive capacity is greatly reduced in the presence of physiological salines. This is an important aspect, in view of the uses of these polymers in diapers and personal hygiene applications. Additional drawbacks of these resins include cumbersome processes of syntheses in some cases and low heat resistance and rapid decay in other instances.
Mixtures of .alpha.-amino acids can be thermally polymerized into proteinoids (Fox and Harada, Science, vol. 128, p. 1214 (1958) and J. Am. Chem. Soc., vol. 82, pp. 3745-3751 (1959)) at temperatures above 150.degree. C. The syntheses require the presence of excess dicarboxylic amino acids for unknown reasons. However, the advantage of excess dicarboxylic amino acid is lost above 210.degree. C., with thermal decomposition of the amino acids. High reproducibility in copolymerization of amino acids has been reported (Fox and Windsor, International Journal of Quantum Chemistry: Quantum Biology, vol. 11, pp. 103-108 (1984)).
However, there are no reports of polyanionic polymers that have specific amino acids incorporated into them that effectively cross-link the polymer during the thermal polymerization or provide sites for post-synthesis chemical crosslinking, or methods for preparing such polymers. In particular, there are no reports of conditions that result in the formation of an insoluble product, capable of absorbing large amounts of water.
Thus, there remains a need for new molecules that function as superabsorbents and a method of preparing such compounds.